A voltage generator with a current limiting is used for an electronic measuring instrument or a semiconductor testing device. As a load, an input terminal, an IO terminal, a supply power terminal, and the like, of an electronic component or a device to be tested are included, and they are used for limiting a current to be equal to or less than a specified current to prevent unnecessary stress.
FIG. 6 illustrates an example of the configuration of a conventional voltage generator with a current limiting function, disclosed in Patent Document 1.
As illustrated in FIG. 6, the voltage generator includes an operational amplifier 1 for buffering a current, an input resistance 2, an output resistance 3 for limiting a current, a feedback resistor 4, a clamper 5 for limiting a current, and a clamper 6, and a positive or negative input voltage Vin which can be varied arbitrarily is applied to an input terminal 7. A load 9 is connected to a load connecting terminal 8.
The clamper 5 consists of two circuits in which a plurality of diodes are connected in series, and these circuits are inversely connected in parallel with each other. In the clamper 6, two diodes are inversely connected in parallel with each other.
Next, an example of the operation of the voltage generator with a current limiting function configured in this way will be described with reference to FIG. 6.
A current limiting operation when positive voltage is generated and a load current Iout flows into the load 9 (a case of a source current), will be described.
Now, suppose that a predetermined load voltage Vout is fed to the load 9, and load a current Iout is given. At that time, suppose that the input voltage Vin fed to the input terminal 7 is a positive voltage Vout. If the load current Iout is within the current limiting range, the input voltage Vin is transmitted directly to the operational amplifier 1 by the feedback resistor 4, and Vin=V2=Vout is satisfied. If the load current Iout is flown out from the operational amplifier 1 to the load 9 through the output resistance 3, voltage drop occurs across the both ends of the output resistance 3, and V3>Vout is satisfied. If the load current Iout is within the current limiting range, the output voltage V3 of the operational amplifier 1 arises, but by the feedback resistor 4, Vin=Vout is satisfied.
As the load current Iout approaches to the current limit value, V3−Vout>n×VF is satisfied. Where, VF is a voltage (forward voltage) at which the current of the diodes begins to flow, and in case of a silicone diode, it is around about 0.6 V. n is the number of the diodes connected to the clamper 5 in series. For example, when the number of diodes is two, the above formula becomes 2×VF, that is about 1.2 V. In this manner, as the load current Iout approaches to the current limit value, the forward diode in the clamper 5 becomes an on state, a current i1 begins to flow in the arrow direction in FIG. 6 and flows into the output terminal 8 through the feedback resistor 4, and a negative input terminal voltage V1 of the operational amplifier 1 rises.
As a result, descent of the output voltage V3 of the operational amplifier 1 causes a current to be limited. Since the load voltage Vout descends being accompanied with this, Vin>Vout is satisfied, and a current i2 also flows into the diodes of the clamper 6 in the direction of the arrow. By the current i2 flowing from the input terminal 7 through the input resistance 2, voltage drop occurs, and the positive input terminal voltage V2 of the operational amplifier 1 descends. The descend of the positive input terminal voltage V2 causes the load voltage Vout to descend and the load current Iout to be limited.
Next, the operation when the load current Iout flows into the load connecting terminal 8 (in case of a sink current) is in a case in which the input voltage Vin of the input terminal 7 is a negative voltage, and in this case, the current direction becomes inverse with respect to the direction of the above description but the operation becomes the same as that of the above description.
Patent Document 1: JP2002-123320A
Incidentally, the voltage generator with a current limiting function disclosed in the Patent Document 1 has the following drawbacks. (1) As illustrated in FIG. 7A, in the clamper 5, diodes are connected in multiple stages. For this reason, as illustrated in FIG. 7B, the current-voltage characteristic (hereinafter, referred to as an I/V characteristic) becomes a gently-sloping curve without abrupt current change at a certain voltage.
Accordingly, since in a conventional voltage generator, current is not abruptly limited to a predetermined value when a current value becomes the predetermined value, it is not desirable for application requiring an exact current limitation.
(2) The forward voltage of the diodes constituting the clamper 5 has large temperature dependence. In case of a usual silicon diode, the forward voltage decreases by an order of several mVs per 1° C. For this reason, the conventional voltage generator has drawbacks in that the current value to be limited has large temperature dependence.(3) In the conventional voltage generator, the I/V characteristic of the clamper 5 becomes as illustrated in FIG. 7B, thereby, the I/V characteristic is determined by the number of connection stages of the diodes. In other words, since the current limit value becomes a fixed value determined by the design value of a circuit, there is a drawback in that a desired current limit value cannot be set.
Therefore, in view of the above-mentioned point, an object of the present invention is to provide a voltage generator with the current limiting function enabling a current limit value to be set arbitrarily, and having a good current limiting characteristic and small temperature dependence of the current limitation, and a semiconductor testing device to which the voltage generator is applied.